Knives of the kind set forth hereinbefore are used in many cases for cutting materials, such as for example paper, cardboard, film, textiles, and the like. A blade, which is generally in the form of a strip steel interchangeable blade with a straight cutting edge, is arranged in that case on a blade slider. The blade slider is displaceable by means of a slider member or handle from outside the housing in such a way that the blade is moved in a front portion out of the housing and can thus be used for cutting. The slider member is generally of such a configuration that the slider member can be actuated by a user with a thumb or index finger. Then, after the slider member is released, the blade is automatically pulled back into the housing again by means of a return element acting on the slider member or the blade slider. An earlier knife of that kind is known, for example, from DE 36 22 342 A1. In that knife, the slider member is connected to the blade slider in such a way that a user has to release the actuating device so that the blade automatically moves back into the housing again.
Besides those knives in which the blade slider is rigidly connected to the slider member, such knives are also known that have a triggerable mechanism which, when triggered by a cutting reaction force, uncouples the blade slider and the slider member so that the blade slider automatically moves back into the housing after the cutting reaction force ceases. Cessation of the cutting reaction force can be afforded, for example, by sliding the knife off the material to be cut. Knives of that kind are therefore also referred to as safety knives.
A safety knife of that kind is known, for example, from DE 19 723 279 C1. In that safety knife, the blade carrier can be coupled to the actuating device in the form of a slider member, by means of a flexible plate, in such a way that the blade carrier and the slider member are coupled upon being extended. In the cutting operation, a cutting reaction force acts on the blade, which is rigidly connected to the blade slider, and that cutting reaction force causes an additional relative movement of the blade and the blade slider in the extension direction relative to the slider member. By virtue of that relative movement of the blade slider, the blade slider is uncoupled from the slider member so that the blade slider together with the blade automatically moves back into the housing after the conclusion of the cutting operation by means of the return element, even if the slider member is not released by the user.
A further knife of that kind is known from EP 1 864 766 B1. In that knife, the blade slider is mounted rotatably in the housing in such a way that in the extended position, the blade slider is rotated by virtue of the cutting reaction force, whereby a rear portion of the blade slider, that is opposite to the blade, comes out of engagement with a portion of the actuating device which is in the form of the slider member, and automatically moves back into the housing after the conclusion of the cutting operation. In addition, a locking spring portion is arranged on the end of the blade slider opposite to the blade, which passes over a rib on the housing when the blade slider is rotated so that, in order to safely slide back into the housing, the blade slider is held in the rotated position by means of that rib and cannot come into engagement again with the actuating device when the cutting reaction force ceases to be applied.
In both known safety knives, uncoupling of the blade slider and the actuating device requires a relatively large amount of force or cutting reaction force, and thus cutting force. In the case of the knife known from DE 19 723 279 C1, that cutting force is the frictional force between the blade and the material to be cut, while in the case of the knife disclosed in EP 1 864 766 B1 this force is the force with which the knife is pressed against the material. Therefore, with both knives the safety aspect can be improved when cutting thin materials, as the triggerable mechanism, which provides that the blade slider and the actuating device are uncoupled, is not triggered because of an inadequate cutting reaction force.